Description: Premixes for medicated feedstuffs are considered to be veterinary medicinal products (VMPs) prepared in advance with a view to the subsequent manufacture of medicated feedstuffs. Medicated feedstuffs should be prepared only from market authorized premixes and premixes for medicated feedstuffs can be used only as prescribed medicines. Apart from active substances, premixes contain carriers which have a role in the homogenization of medicated feedstuffs. Wheat bran as a carrier may be a source of different, potentially harmful or toxic substances e.g. aflatoxin. In our study, 15 different batches of premixes with a wheat bran component were tested for aflatoxin B1 (AFB1) by means of an enzyme-linked immunosorbent assay (ELISA). The samples tested showed contamination with AFB1 ranging from 1.5 to 35 ng/g (mean = 18.79 ng/g). However, no correlation between AFB1 levels and the month of collection or season of production was observed. Considering the composition of the premix, the only possible source of ontamination with AFB1was wheat bran. Contamination probably occurred before the production of the premix, probably in the fi eld orduring storage. Concentrations of active substances and citric acid (a neutralising agent for mycotoxins) were in accordance with the producer`s declaration. Our discovery of AFB1 in the tested premixes was in contravention of the definition and primary role of veterinary medicinal products. In this study, we would like to highlight the need for monitoring raw materials of biological origin for premix production. Even though carriers are not pharmacologically active substances, an efficient method for controlling potential contaminants such asmycotoxins or their toxic components should be proposed, with the aim of protecting animal health, onsumers, employees and end-users in the production of veterinary medicinal products